Why my feiend converted to Islam, he call himself Yusof?

Many of my friend converted to Christianity but the name no need to change?

In Protestant Christianity baptism, taking a Christian name is optional. In Catholicism, I understand that baptism at birth, Christian name is given by the parents; baptism at confirmation or conversion, Christian name is given by the priest. In conversion to Islam, I understand that being given or taking on a Muslim name at birth or at conversion is compulsory. And for conversion, even the surname is usually changed to Abdullah (meaning child of Allah). Anyway, many Christian and Muslim names share common historical roots and references, for example:

Joseph = Yusoff
Abraham = Ibrahim
Jacob = Yaccob
Issac = Ishak
David = Daud
Solomon = Sulaiman